Nondestructive Evaluation (NDE) uses tests to examine an object or material to detect imperfections, determine properties, or asses quality without changing its usefulness. NDE methods have been developed over the years using high powered optical techniques, ultrasonic surface wave measurement, stress fluorescent, penetrant techniques, and eddy current inspection analysis. This invention is primarily concerned with utilizing an eddy current imaging system for the detection of laser generated thermal and elastic strains in materials for advanced material characterization. This advanced technology can be used for surface displacement evaluation, defect detection and damage assessment.
Eddy-current probes have been used in the past as a form of NDE and recently in conjunction with photoinductive imaging. Photoinductive imaging is a unique dual-mode NDE technique that combines eddy current and thermal wave methods. The photoinductive effect, upon which this method is based, is the thermally induced change in the impedance of an eddy current probe in proximity to a conducting surface that is illuminated with a modulated light source. The change in probe impedance is caused by the temperature-induced changes in the conductivity and permeability of the specimen. Typical changes in probe impedance are small, on the order of a few ppm, but because they are synchronous with the light-beam modulation, lock-in techniques can be used to detect the signals, which can then be used to image surface or near-surface defects, voids, inclusions, or other thermal or structural inhomogeneities. The problems with the prior art photoinductive NDE has been that they offer only one side excitation and detection on the specimen and can only be used to examine very thin surface features within sub-microns.